Emacs: Universal Argument (prefix arg)

What is Universal Argument

universal-argument 【Ctrl+u】 allows you to give argument to commands, interactively.

What the command do with argument depends on the command. It may repeat the command, or have different behavior (e.g. copy file name without dir path, or copy full path.) .

Example of universal-argument

• Type Ctrl+u 60 - to insert hyphen, 60 times.

• In dired, dired-copy-filename-as-kill 【w】 copies the file name, but type Ctrl+u 0 first, then w, the copied name is file full path.

• with Ctrl+u, shell starts a new buffer

• with Ctrl+u, shell-command-on-region replace region with output.

digit-argument, negative-argument

These are equivalent of universal-argument.

• digit-argument is equivalent to calling universal-argument 【Ctrl+u】 then type a number.
• negative-argument is equivalent to calling universal-argument 【Ctrl+u】 then type a negative number.

Keyboard shortcuts for digit-argument

• The keys Ctrl+1 to Ctrl+9 and Meta+1 to Meta+9 are bound to digit-argument.
• The key Ctrl+- and Meta+- calls negative-argument.